1. Field of the Invention
The present invention relates to a top bracing of a marine engine, and more particularly, the present invention relates to a top bracing of a marine engine, which is used to connect an upper end of the marine engine to a hull structure for damping vibration generated by the marine engine.
2. Description of the Related Art
Generally, in a ship, a great amount of vibration is generated while a marine engine runs. The vibration of the is marine engine accelerates fatigue fracturing of the marine engine itself and its surrounding components and deteriorates on-board comfortableness. Therefore, because the vibration of the marine engine acts as an undesirable factor, wherever possible, it is preferred to damp the vibration. As means for damping the vibration of the marine engine, a method for connecting an upper end of the marine engine to a hull structure by using a top bracing is widely used.
FIG. 6 is a schematic longitudinal cross-sectional view of a ship, illustrating an installation position of a top bracing for a marine engine. As shown in FIG. 6, a marine engine 3 of a ship 1 is fixedly mounted on an engine mounting 4. In this state, if the engine 3 runs, an upper end of the engine 3 is fiercely vibrated. Accordingly, in the ship 1, a top bracing 100 is installed between the hull structure 2 and the engine 3 to damp the vibration of the engine 3.
FIG. 7 is a front view illustrating a conventional top bracing for a marine engine; and FIG. 8 is a plan view of the conventional top bracing for a marine engine of FIG. 7. As shown in FIGS. 7 and 8, a conventional top bracing 100 includes a pair of engine connection plates 110 which are connected to the engine 3, a pair of hull connection plates 120 which are connected to the hull structure 2, and a connection beam 150 which connects the pair of engine connection plates 110 and the pair of hull connection plates 120 with each other.
The hull connection plates 120 are coupled to a projection plate 140 of the hull structure 2 via a pair of friction plates 130 by bolts 141 and nuts 142. The friction plates 130 allow the pair of hull connection plates 120 to be moved to some extent. The extent to which the hull connection plates 120 are moved, can be regulated by tightening or loosening the bolts 141.
Although the conventional top bracing 100 constructed as mentioned above can effectively damp transverse vibration of the marine engine 3, it cannot damp in a sufficient manner multi-directional stress, such as torsional stress, due to relative height variations between the hull structure 2 and the engine 3 which can be caused by a change in wave or draft, etc. while the ship 1 is afloat.
Especially, connecting portions among the hull structure 2, engine 3, engine connection plates 110, hull connection plates 120, projection plate 140 and connection beam 150 are likely to be fatigued and thereby to produce cracks, etc. due to the presence of residual stress, material deformation, weld defects, etc. which are created in the course of welding. Once cracks are produced in one of the components, other components are also consecutively cracked to further develop the vibration, by which the likelihood of the engine 3 to be adversely influenced is increased, and according to this, repairing and replacing operations for the top bracing 10 must be frequently performed, which are troublesome.
In other words, in the case that the conventional top bracing 100 is used, when considering a graph illustrating a vibration response curve which representatively reveals engine vibration characteristics and shows a relationship between engine rpm and engine vibration, as shown in FIG. 9, the vibration response curve has one apex. Here, the apex of the vibration response curve represents engine rpm which corresponds to a natural frequency of the engine. Once the conventional top bracing 100 is mounted to the engine 3, because its natural frequency f0 is fixedly determined and unchanged, as engine rpm approaches to the natural frequency f0, the vibration is severely generated, thereby causing some or all of the above-mentioned problems.
Furthermore, in the connecting portions for the hull structure 2 of the conventional top bracing 100, because the hull connection plates 120 are connected to the hull structure 2 via the friction plates 130 and projection plate 140 by bolts 141 and nuts 142, the hull connection plates 120 have some flexibility which acts against the vibration of the engine. However, in the connecting portions for the engine 3 of the conventional top bracing 100, because the connection beam 150 is directly jointed through the engine connection plates 110 to the engine 3 by welding, a problem is caused in that it is impossible to flexibly support the engine 3.
Besides, while it is the norm that a top bracing is prefabricated on the ground and then moved onto a ship thereby to be mounted, in the case of the conventional top bracing 100, once the top bracing 100 is prefabricated, it is difficult to adjust a length of the top bracing 100. Therefore, when the top bracing 100 is prefabricated on the ground, it is necessary to accord with a high precision the length of the top bracing 100 to a length measured between the hull structure 2 and the engine 3, thereby to ease mounting operations for the top bracing 100. Nevertheless, since the length between the hull structure 2 and the engine 3 can vary to some extent with an error, difficulties are caused in precisely according the length of the top bracing 100 to the length measured between the hull structure 2 and the engine 3 when prefabricating the top bracing 100 on the ground.
By reason of this, when the conventional top bracing 100 is mounted to the engine 3, a length of the top bracing 100 must be frequently readjusted such that it corresponds to an actual length between the hull structure 2 and the engine 3, which may turn out to be cumbersome and in the course of which weld defects may be created.